Isolation of novolak resin without high temperature distillation and photoresist composition therefrom

ABSTRACT

The present invention provides a method for producing a water insoluble, aqueous alkali soluble novolak resins having consistent molecular weight and superior performance in photoresist composition, by isolating such novolak resin without high temperature distillation. A method is also provided for producing photoresist composition from such a novolak resin and for producing semiconductor devices using such a photoresist composition.

This application is a division of application Ser. No. 08/939,253, filed Sep. 29, 1997, now U.S. Pat. No. 5,863,700, which in turn was a continuation of application Ser. No. 08/568,916, filed Dec. 7, 1995, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a process for producing a novolak resin having consistent molecular weight and high performance, and for using such a novolak resin in light-sensitive compositions. The present invention also relates to a process for making a superior quality light-sensitive composition useful as a positive-working photoresist. Further, the present invention relates to a process for coating substrates with these light-sensitive compositions, as well as the process of coating, imaging and developing these light-sensitive mixtures on such substrates.

Photoresist compositions are used in microlithography processes for making miniaturized electronic components, such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin coating of a film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure to radiation.

This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate.

Novolak resins are frequently used a polymeric binder in liquid photoresist formulations. These resins are typically produced by conducting a condensation reaction between formaldehyde and one or more multi-substituted phenols, in the presence of an acid catalyst, such as oxalic acid, maleic acid, or maleic anhydride. In producing sophisticated semiconductor devices, it has become increasingly important to provide novolak resin of superior quality in terms of dissolution rate, better binding properties with diazonaphthoquinone, and heat resistance.

There are two types of photoresist compositions, negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to such a solution. Thus, treatment of an exposed negative-working resist with a developer causes removal of the non-exposed areas of the photoresist coating and the creation of a negative image in the coating. Thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited.

On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the photoresist composition exposed to the radiation become more soluble to the developer solution (e.g. a rearrangement reaction occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working photoresist with the developer causes removal of the exposed areas of the coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying substrate surface is uncovered.

After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution or plasma gases and the like. The etchant solution or plasma gases etch that portion of the substrate where the photoresist coating was removed during development. The areas of the substrate where the photoresist coating still remains are protected and, thus, an etched pattern is created in the substrate material which corresponds to the photomask used for the image-wise exposure of the radiation. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface. In some instances, it is desirable to heat treat the remaining photoresist layer, after the development step and before the etching step, to increase its adhesion to the underlying substrate and its resistance to etching solutions.

Positive working photoresist compositions are currently favored over negative working resists because the former generally have better resolution capabilities and pattern transfer characteristics. Photoresist resolution is defined as the smallest feature which the resist composition can transfer from the photomask to the substrate with a high degree of image edge acuity after exposure and development. In many manufacturing applications today, resist resolution on the order of less than one micron are necessary. In addition, it is almost always desirable that the developed photoresist wall profiles be near vertical relative to the substrate. Such demarcations between developed and undeveloped areas of the resist coating translate into accurate pattern transfer of the mask image onto the substrate.

DESCRIPTION OF THE PRIOR ART

In the recent years there has been significant progress in novolak resin synthesis. It has been reported that under vigorous synthetic conditions the structure of novolak resin changes, especially when high concentration of acid catalyst and high temperature is used, Rahman et al, "Rearrangement of Novolak Resin", presented at SPIE conference, 1994; Khadim et al "The nature and Degree of Substitution Patterns in Novolaks by Carbon-13 NMR Spectroscopy", presented at SPIE conference, 1993. In a typical novolak reaction, a reactor is charged with phenolic compounds, an acid catalyst such as oxalic acid, maleic acid, p-toluene sulphonic acid or any mineral acid, and heated to about 95 to 100° C. Formaldehyde is slowly added and the mixture is heated at reflux for about 6 hours. At the end of the condensation period, the reactor is converted to distillation, and the temperature is raised to about 200° C. At this point vacuum is slowly drawn, the temperature is raised to about 220° C., and the pressure is reduced to below about 20 mm Hg. After the volatiles have been distilled off, the vacuum is released and the molten resin collected and allowed to cool. During the course of the above resin synthesis sequence, samples were taken at various temperatures and inspected by GPC (Gas Phase Chromotography). It was found that there was a decrease of the weight average molecular weight (all molecular weights are weight average unless otherwise specified) of the polymer, especially in the temperature range between about 160-190° C. (Rahman et al, "The Effect of Lewis Bases on the Molecular Weight of Novolak Resins", presented at Ellenville Conference, 1994). The molecular weight decrease was not observed unless the phenolic compounds are extremely pure. If the phenolic compounds contains a trace amount of nitrogen base, the molecular weight decrease during the distillation process was not observed. In copending U.S. patent applications Ser. No. 997,942, filed Dec. 29, 1992 (WO 94/14862) and Ser. No. 999,500, filed Dec. 29, 1992 (WO 94/14863), assigned to the same assignee as the subject application and incorporated herein by reference, an improved process is disclosed to control molecular weight by adjusting the amount of Lewis base in the phenolic compounds before or after the condensation. It was disclosed that during the purification process of such phenolic compounds using an ion exchange resin, distillation, and/or a solvent extraction process, to remove metal ions, a minor amount of base present was also removed. Due to the absence of this base, the novolak resin was partially depolymerized during the manufacturing process. The physical properties of the depolymerized resin changed due to degradation and it was not useful for photoresist compositions. This problem can be substantially avoided by adjusting the level of Lewis base before or after the condensation step of the novolak manufacturing process.

In copending U.S. patent application Ser. No. 366,634, filed on Dec. 30, 1994, assigned to the same assignee as the subject application and incorporated herein by reference, an improved process is disclosed for isolating a novolak resin at a temperature less than about 140° C. by using subsurface forced steam distillation to avoid high temperature molecular weight breakdown of the resin. It is known that an alkali soluble novolak resin can be made by the condensation reaction of a mixture of phenolic monomers with an aldehyde source. Such novolak resin synthesis processes are disclosed in U.S. Pat. No. 5,346,799, incorporated herein by reference.

SUMMARY OF THE INVENTION

It has now been found that the novolak resin synthesis process can be further improved by isolating the phenol formaldehyde condensation product at room temperature without any high temperature distillation. This can be accomplished by adding a water soluble organic solvent, such as acetone or a C₁ -C₃ alkyl alcohol, such as methanol, to the phenol-formaldehyde condensation product, followed by addition of water, preferably DI (deionized) water, to thereby precipitate the condensation product, leaving behind the unreacted phenols, and useless oligomers in the water soluble solvent/water solution. This precipitate can then be dissolved in a suitable novolak resin solvent such as PGMEA (propylene glycol methyl ether acetate), 2-heptanone or ethyl lactate, or it can be dried, such as in a vacuum drier.

The present invention relates to a process for producing novolak resin of consistent molecular weight having high performance and for using such a novolak resin in light-sensitive compositions. The invention also relates to a photoresist composition containing such a novolak resin and to a process for producing such photoresist compositions. The invention further relates to semiconductor devices using such photoresists containing these novolak resin and a photosensitizer, and to a process for using such photoresists in producing semiconductor devices.

Water insoluble, aqueous alkali soluble, film forming novolak resins having a consistent molecular weight and high performance may be obtained by condensing formaldehyde having a very low level of metal ions with one or more phenolic compounds, such as m-cresol, p-cresol, 2,4 and 2,5-dimethylphenol, 3,5-dimethylphenol, and 2,3,5-trimethylphenol, having a very low level of metal ions. The condensation reaction is preferably carried out in the presence of an acid catalyst, such as oxalic acid, maleic acid, maleic anhydride, p-toluene sulphonic acid or sufuric acid.

In the process of the present invention, the precipitate is either dissolved in a novolak resin solvent and the residual water is removed, such as by low temperature vacuum distillation, to obtain a novolak resin having a very consistent molecular weight with superior performance in photoresist compositions. The precipitate may also be dried, such as in a vacuum oven, to remove residual water and obtain such a novolak resin.

The present invention provides a process which comprises:

a) condensing formaldehyde with one or more phenolic compounds, in the presence of an acid catalyst, and after the condensation reaction is complete, adding a water soluble solvent, such as acetone or a C₁ -C₃ alkyl alcohol, such as methanol;

b) precipitating the condensation product by adding water, preferably DI water, and removing the liquid, such as by decanting, and thereby also removing the unreacted phenolic compounds;

c) dissolving the precipitate in a suitable novolak resin solvent ,and removing the residual water, such as by low temperature vacuum distillation, thereby producing a water insoluble, aqueous alkali soluble, film forming novolak resin.

The present invention further provides a process for producing a positive photoresist composition having a superior performance. The subject process comprises:

a) condensing formaldehyde with one or more phenolic compounds, in the presence of an acid catalyst, and after the condensation reaction is complete, adding a water soluble solvent, such as acetone or a C₁ -C₃ alkyl alcohol, such as methanol;

b) precipitating the condensation product by adding water, preferably DI water, and removing the liquid, such as by decanting, and thereby also removing the unreacted phenolic compounds;

c) dissolving the precipitate in a suitable novolak resin solvent, and removing the residual water, such as by low temperature vacuum distillation, thereby producing a water insoluble, aqueous alkali soluble, film forming novolak resin.

d) providing an admixture of: 1) a photosensitive component in an amount sufficient to photosensitive the photoresist composition; 2) the water insoluble, aqueous alkali soluble, film forming novolak resin from step c); and 3) a suitable photoresist solvent, and thereby forming a photoresist composition.

The invention further provides a method for producing a semiconductor device by producing a photo-image on a substrate by coating a suitable substrate with a positive working photoresist. The subject process comprises:

a) condensing formaldehyde with one or more phenolic compounds, in the presence of an acid catalyst, and after the condensation reaction is complete, adding a water soluble solvent, such as acetone or a C₁ -C₃ alkyl alcohol, such as methanol;

b) precipitating the condensation product by adding DI water, and removing the liquid, such as by decanting, and thereby also removing the unreacted phenolic compounds;

c) dissolving the precipitate in a suitable novolak resin solvent and removing the residual water, such as by low temperature vacuum distillation, thereby producing a water insoluble, aqueous alkali soluble, film forming novolak resin having a consistent molecular weight and superior quality;

d) providing an admixture of: 1) a photosensitive component in an amount sufficient to photosensitive the photoresist composition; 2) the water insoluble, aqueous alkali soluble, film forming novolak resin and 3) a suitable photoresist solvent and thereby forming a photoresist composition;

e) coating a suitable substrate with the photoresist composition;

f) heat treating the coated substrate until substantially all of the photoresist solvent is removed; image-wise exposing the photosensitive composition and removing the image-wise exposed areas of such composition with a suitable developer, such as an aqueous alkaline developer. Optionally one may also perform a baking of the substrate either immediately before or after the removing step.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Novolak resins have been commonly used in the art of photoresist manufacture as exemplified by "Chemistry and Application of Phenolic Resins", Knop A. And Scheib, W.; Springer Verlag, New York, 1979 in Chapter 4. Similarly, o-quinone diazides are well known to the skilled artisan as demonstrated by "Light Sensitive Systems", Kosar, J.; John Wiley & Sons, New York, 1965 Chapter 7.4. However, the instant invention has found that the use of particular resins isolated without high temperature distillation, as opposed to those taught in the prior art, produces a photoresist having a superior resolution and depth of focus.

The present invention provides a novolak resin, a photoresist composition containing such a novolak resin and a process for producing semiconductor devices using such a photoresist composition. The photoresist composition is formed by providing an admixture of a photosensitizer, the subject water insoluble, aqueous alkali soluble novolak resin and a suitable photoresist solvent. Suitable solvents for such photoresists and for novolak resins may include propylene glycol mono-alkyl ether, propylene glycol alkyl (e.g. methyl) ether acetate, ethyl-3-ethoxypropionate, ethyl lactate, mixtures of ethyl-3-ethoxypropionate and ethyl lactate, butyl acetate, xylene, diglyme, ethylene glycol monoethyl ether acetate. The preferred solvents are propylene glycol methyl ether acetate (PGMEA), ethyl lactate, 2-heptanone, and ethyl-3-ethoxypropionate (EEP).

Other optional ingredients such as colorants, dyes, anti-striation agents, leveling agents, plasticizers, adhesion promoters, speed enhancers, solvents and such surfactants as non-ionic surfactants may be added to the solution of novolak resin, sensitizer and solvent before the photoresist composition is coated onto a substrate. Examples of dye additives that may be used together with the photoresist compositions of the present invention include Methyl Violet 2B (C.I. No. 42535), Crystal Violet (C.I. 42555). Malachite Green (C.I. No. 42000), Victoria Blue B (C.I. No. 44045) and Neutral Red (C.I. No. 50040) at one to ten percent weight levels, based on the combined weight of novolak and sensitizer. The dye additives help provide increased resolution by inhibiting back scattering of light off the substrate.

Anti-striation agents may be used at up to about a five percent weight level, based on the combined weight of novolak and sensitizer. Plasticizers which may be used include, for example, phosphoric acid tri-(beta-chloroethyl)-ester; stearic acid; dicamphor; polypropylene; acetal resins; phenoxy resins; and alkyl resins, at about one to ten percent weight levels, based on the combined weight of novolak and sensitizer. The plasticizer additives improve the coating properties of the material and enable the application of a film that is smooth and of uniform thickness to the substrate.

Adhesion promoters which may be used include, for example, beta-(3,4-epoxy-cyclohexyl)-ethyltrimethoxysilane; p-methyl-disilane-methyl methacrylate; vinyl trichlorosilane; and gamma-amino-propyl triethoxysilane, up to about a 4 percent weight level, based on the combined weight of novolak and sensitizer. Development speed enhancers that may be used include, for example, picric acid, nicotinic acid or nitrocinnamic acid up to about a 20 percent weight level, based on the combined weight of novolak and sensitizer. These enhancers tend to increase the solubility of the photoresist coating in both the exposed and unexposed areas, and thus they are used in applications when speed of development is the overriding consideration even though some degree of contrast may be sacrificed; i.e., while the exposed areas of the photoresist coating will be dissolved more quickly by the developer, the speed enhances will also cause a larger loss of photoresist coating from the unexposed areas.

The solvents may be present in the overall composition in an amount of up to 95% by weight of the solids in the composition. Solvents, of course are substantially removed after coating of the photoresist solution on a substrate and subsequent drying. Non-ionic surfactants that may be used include, for example, nonylphenoxy poly(ethyleneoxy) ethanol; octylphenoxy ethanol at up to about 10% weight levels, based on the combined weight of novolak and sensitizer.

The prepared photoresist solution, can be applied to a substrate by any conventional method used in the photoresist art, including dipping, spraying, whirling and spin coating. When spin coating, for example, the resist solution can be adjusted with respect to the percentage of solids content, in order to provide coating of the desired thickness, given the type of spinning equipment utilized and the amount of time allowed for the spinning process. Suitable substrates include silicon, aluminum, polymeric resins, silicon dioxide, doped silicon dioxide, silicon nitride, tantalum, copper, polysilicon, ceramics, aluminum/copper mixtures; gallium arsenide and other such Group III/V compounds.

The photoresist coatings produced by the described procedure are particularly suitable for application to thermally grown silicon/silicon dioxide-coated wafers, such as are utilized in the production of microprocessors and other miniaturized integrated circuit components. An aluminum/aluminum oxide wafer can also be used. The substrate may also comprise various polymeric resins, especially transparent polymers such as polyesters. The substrate may have an adhesion promoted layer of a suitable composition, such as one containing hexa-alkyl disilazane.

The photoresist composition solution is then coated onto the substrate, and the substrate is treated at a temperature from about 70° C. to about 110° C. for from about 30 seconds to about 180 seconds on a hot plate or for from about 15 to about 90 minutes in a convection oven. This temperature treatment is selected in order to reduce the concentration of residual solvents in the photoresist, while not causing substantial thermal degradation of the photosensitizer. In general, one desires to minimize the concentration of solvents and this first temperature treatment is conducted until substantially all of the solvents have evaporated and a thin coating of photoresist composition, on the order of one micron in thickness, remains on the substrate. In a preferred embodiment the temperature is from about 85° C. to about 95° C. The treatment is conducted until the rate of change of solvent removal becomes relatively insignificant. The temperature and time selection depends on the photoresist properties desired by the user, as well as the equipment used and commercially desired coating times. The coated substrate can then be exposed to actinic radiation, e.g., ultraviolet radiation, at a wavelength of from about 300 nm to about 450 nm, x-ray, electron beam, ion beam or laser radiation, in any desired pattern, produced by use of suitable masks, negatives, stencils, templates, etc.

The photoresist is then optionally subjected to a post exposure second baking or heat treatment either before or after development. The heating temperatures may range from about 90° C. to about 120° C. more preferably from about 100° C. to about 110° C. The heating may be conducted for from about 30 seconds to about 2 minutes, more preferably from about 60 seconds to about 90 seconds on a hot plate or about 30 to about 45 minutes by convection oven.

The exposed photoresist-coated substrates are developed to remove the image-wise exposed areas by immersion in an alkaline developing solution or developed spray development process. The solution is preferably agitated, for example, by nitrogen burst agitation. The substrates are allowed to remain in the developer until all, or substantially all, of the photoresist coating has dissolved from the exposed areas. Developers may include aqueous solutions of ammonium or alkali metal hydroxides. One preferred hydroxide is tetramethyl ammonium hydroxide. After removal of the coated wafers from the developing solution, one may conduct an optional post-development heat treatment or bake to increase the coating's adhesion and chemical resistance to etching solutions and other substances. The post-development heat treatment can comprise the oven baking of the coating and substrate below the coating's softening point. In industrial applications, particularly in the manufacture of microcircuitry units on silicon/silicon dioxide-type substrates, the developed substrates may be treated with a buffered, hydrofluoric acid base etching solution. The photoresist compositions of the present invention are resistant to acid-base etching solutions and provide effective protection for the unexposed photoresist-coating areas of the substrate.

The following specific examples will provide detailed illustrations of the methods of producing and utilizing compositions of the present invention. These examples are not intended, however, to limit or restrict the scope of the invention in any way and should not be construed as providing conditions, parameters or values which must be utilized exclusively in order to practice the present invention.

EXAMPLE 1

250 grams of phenolic compounds consisting of 5.0 moles of m-cresol and 3.0 moles of 3,5-xylenol were transferred to a four necked flask equipped with a condenser, a thermometer, and a dropping funnel. 2.5 grams of oxalic acid (1% by weight of the phenols) was added and the flask was heated to 95° C. 123.0 g of formaldehyde (molar ratio of phenols/formaldehyde 1/0.685) was added dropwise over one and a half hours. The reaction was allowed to continue for 6 hours at 95° C. 600 grams of methanol was added and stirred for 10 minutes. 92.7 grams of DI water (11.1%, by weight, of the batch) was added with stirring over a period often minutes. Stirring was stopped and a white precipitate settled to the bottom of the flask. The liquid was sucked out from the top of a second flask into which the precipitate was placed. The precipitate was collected as fraction A in a beaker and dried in a vacuum oven. 80 grams of water was added to the liquid in the second flask and fraction B was separated in the same manner as fraction A. The process was continued and the fractions C and D were separated in the same manner. All the fractions were then dried in a vacuum drier. The unreacted phenols and very low molecular weight oligomers remained in the methanol water solution. The analytical results are shown in Table 1 below.

                  TABLE 1                                                          ______________________________________                                         Fractions                                                                             Weight (g)                                                                               GPC MW   PD   % m-Cresol                                                                             % 3,5-Xyl                               ______________________________________                                         B      24.87     8733     5.5  2.3     <0.1                                    C      72.43     7158     3.8  2.1     <0.1                                    D      23.49     3162     1.9  1.7     <0.1                                    E      23.37     1701     1.6  2.3     <0.1                                    ______________________________________                                    

EXAMPLE 2

150 grams of phenolic compounds consisting of 6.0 moles of m-cresol, 2.5 moles of 3,5-xylenol and 0.5 moles of 2,3,5-xylenol were transferred to a four necked flask equipped with a condenser, a thermometer, and a dropping funnel. 1.125 grams of oxalic acid (0.75% by weight of the phenols) was added and the flask was heated to 95° C. 70.7 g of formaldehyde (molar ratio of phenols/formaldehyde 1/0.66) was added dropwise over one and a half hours. The reaction was allowed to continue for 6 hours at 95° C. 329 grams of methanol was added and stirred for 10 minutes. 175 grams of DI water (43.7% of the batch) was added and during ten minutes and stirred for 30 minutes. Stirring was stopped and a white precipitate was settled at the bottom of the flask. The liquid was sucked out from the top and discarded. The precipitate was collected in a beaker and dried in a vacuum oven.

EXAMPLE 4

A 50 gram photoresist test sample was prepared according to the following formulation:

    ______________________________________                                         NK-280 (a proprietary 2,1,5-diazonaphthoquinone                                                           2.0218 gm                                           sulfonyl chloride based sensitizer from Nippon Zeon Co)                        NK-240 (a proprietary 2,1,4-diazonaphthoquinone                                                           0.8382 gm                                           sulphonyl chloride based sensitizer from Nippon Zeon Co.)                      Novolak Resin fraction C from example 1                                                                   1.3545 gm                                           Novolak Resin fraction D from example 1                                                                   1.35345 gm                                          Novolak Resin fraction E from example 1                                                                   4.0634 gm                                           Pyrogallol from Aldrich Co.                                                                               0.1630 gm                                           BI26X-SA (a proprietary speed enhancer from Nippon                                                        1.203 gm                                            Zeon Co.)                                                                      KP-341, a striantion free surfactant                                                                      0.004 gm                                            from Shinetsue Chem. Co. (2% in Ethyl Lactate)                                 Ethyl Lactate              33.147 gm                                           n-Butyl Acetate            5.849 gm                                            ______________________________________                                    

The resist sample was coated on a hexamethyldisilazane (HMDS) primed silicon wafer to 1.083μm thickness using a soft bake at 90° C. for 60 seconds on an I-line hot plate (SVG® 8100). The exposure matrix was printed on the coated wafers using a 0.54 NA NIKON® i-line stepper and a NIKON® resolution reticle. The exposed wafers were PEB (post exposure baked) at 110° C. for 70 seconds on a in line hot plate. The wafers were then developed using AZ® 300 MIF TMAH (tetramethyl ammonium hydroxide--2.38%) developer. The developed wafers were examined using a HITACHI® S-400 SEM (scanning electron microscope). A nominal dose (Dose to Print, DTP) is measured at the best focus, the dose required to precisely replicate a given feature. Resolution and depth of focus (DOF) were measured and are shown in Table 2 below.

EXAMPLE 5

A 50 gram photoresist test sample was prepared according to the following formulation:

    ______________________________________                                         NK-280 (a proprietary 2,1,5-diazonaphthoquinone                                                            2.0218 gm                                          sulfonyl chloride based sensitizer from Nippon Zeon Co)                        NK-240 (a proprietary 2,1,4-diazonaphthoquinone                                                            0.8382 gm                                          sulphonyl chloride based sensitizer from Nippon Zeon Co.)                      Novolak Resin fraction D from example 1                                                                    1.437 gm                                           Novolak Resin fraction E from example 1                                                                    5.336 gm                                           Pyrogallol from Aldrich Co. 0.1630 gm                                          BI26X-SA (a proprietary speed enhancer from Nippon                                                         1.203 gm                                           Zeon Co.)                                                                      KP-341, a striantion free surfactant                                                                       0.004 gm                                           from Shinetsue Chem. Co. (2% in Ethyl Lactate)                                 Ethyl Lactate               33.147 gm                                          n-Butyl Acetate             5.849 gm                                           ______________________________________                                    

The photoresist resist sample was coated on a hexamethyldisilazane (HMDS) primed silicon wafer to 1.083μm thickness using a soft bake at 90° C. for 60 seconds on an I-line hot plate (SVG® 8100). The exposure matrix was printed on the coated wafers using a 0.54 NA NIKON® i-line stepper and a NIKON® resolution reticle. The exposed wafers were PEB (post exposure baked) at 110° C. for 70 seconds on a in line hot plate. The wafers were then developed using AZ® 300 MIF TMAH (tetramethyl ammonium hydroxide--2.38%) developer. The developed wafers were examined using a HITACHI® S-400 SEM (scanning electron microscope). A nominal dose (Dose to Print, DTP) is measured at the best focus, the dose required to precisely replicate a given feature. Resolution and depth of focus (DOF) were measured and are shown in Table 2 below.

                  TABLE 2                                                          ______________________________________                                         Example #                                                                             Resin From      DTP     Resolution                                                                            DOF                                      ______________________________________                                         4      Example 1; fractions C,                                                                        105     0.36   (-.6/.4)                                        D, & E                                                                  5      Example 1; fractions D & E                                                                      80     0.36   (-.6/.4)                                 ______________________________________                                    

COMPARATIVE EXAMPLE 1

250 grams of phenolic compounds consisting of 5.0 moles of m-cresol and 3.0 moles of 3,5-xylenol were transferred to a four necked flask equipped with a condenser, a thermometer, and a dropping funnel. 2.5 grams of oxalic acid (1% by weight of the phenols) was added and the flask was heated to 95° C. 123.0 g of formaldehyde (molar ratio of phenols/formaldehyde 1/0.685) was added dropwise over one and a half hours. The reaction was allowed to continue for 6 hours at 95° C. The reaction mixture was distilled up to 190° C. to remove water and solvent, and then distilled under vacuum to 210° C. to remove unreacted phenols. The molted resin was collected in an aluminum tray. A photoresist formulation was made exactly as in example #4, except novolak resin is used from comparative example #1 and studied as example #4. It was found that the performance of the resist(Table 3) is not good as compare to example #4 and #5.

                  TABLE 3                                                          ______________________________________                                         Example #                                                                               DTP      Resolution      DOF                                          ______________________________________                                         Comparative                                                                             160      Pattern falling over at 0.38                                                                   (-.6/-0.8)                                   example #1        micron                                                       ______________________________________                                     

What is claimed is:
 1. A method for producing a water insoluble, aqueous alkali soluble novolak resin consisting essentially of:a) condensing formaldehyde with one or more phenolic compounds, in the presence of an acid catalyst and, after the condensation reaction is complete, adding a water soluble solvent; b) precipitating the condensation product from step a) by adding water, and removing the unreacted phenolic compounds and oligomers in a water soluble solvent/water solution, and thereby removing the unreacted phenolic compounds and oligomers with said solution; c) dissolving the precipitate from step b) in a novolak resin solvent selected from the group consisting of propylene glycol methyl ether acetate, 2-heptanone, ethyl lactate, ethyl-3-ethoxypropionate, butyl acetate, xylene, diglyme, and ethylene glycol monoethyl ether acetate, and removing the residual water, thereby producing a water insoluble, aqueous alkali soluble, film forming novolak resin.
 2. The method of claim 1 where in said acid catalyst is oxalic acid, maleic acid, maleic anhydride, sulfuric acid or p-toluene sulphonic acid.
 3. The method of claim 1 wherein the water soluble solvent is acetone or a C₁ -C₃ alkyl alcohol.
 4. The method of claim 1 wherein the water soluble solvent is methanol.
 5. A method for producing a semiconductor device by producing a photo-image on a suitable substrate consisting essentially of:a) condensing formaldehyde with one or more phenolic compounds, in the presence of an acid catalyst and, after the condensation reaction is complete, adding a water soluble solvent; b) precipitating the condensation product from step a) by adding water, and removing the unreacted phenolic compounds and oligomers in a water soluble solvent/water solution, and thereby removing the unreacted phenolic compounds and oligomers with said solution; c) dissolving the precipitate from step b) in a novolak resin solvent selected from the group consisting of propylene glycol methyl ether acetate, 2-heptanone, ethyl lactate, ethyl-3-ethoxypropionate, butyl acetate, xylene, diglyme, and ethylene glycol monoethyl ether acetate, and removing the residual water, thereby producing a water insoluble, aqueous alkali soluble, film forming novolak resin; d) providing an admixture of: 1) a photosensitive component in an amount sufficient to photosensitize the photoresist composition; 2) the water insoluble, aqueous alkali soluble, film forming novolak resin from step c) and 3) a suitable photoresist solvent, and thereby forming a photoresist composition; e) coating a suitable substrate with the photoresist composition from step d); f) heat treating the coated substrate from step e) until substantially all of the photoresist solvent is removed; image-wise exposing the photoresist composition and then removing the image-wise exposed areas of said photoresist composition with a suitable developer; optionally one may also perform a baking of the substrate either immediately before or after the removing step.
 6. The method of claim 5 wherein the acid catalyst is oxalic acid, maleic acid, maleic anhydride, sulfuric acid, or p-toluene sulphonic acid.
 7. The method of claim 5 wherein the photoresist solvent comprises propylene glycol methyl ether acetate, 2-heptanone, ethyl lactate, or ethyl-3-ethoxypropionate.
 8. The method of claim 5 wherein the water soluble solvent is acetone or a C₁ -C₃ alkyl alcohol.
 9. The method of claim 5 wherein the water soluble solvent is methanol. 